I had a TPO flat roof that was not pitched or drained properly, so there was always standing water. Other companies wanted to frame up the corners with "crickets" to try to force the water to the drains which were not properly recessed lower in the roof like a shower drain is. The other guys were more expensive and they wouldn't even guarantee it would eliminate standing water. Eli and team suggested just re doing the drains after their inspection. He said the roof itself was fine, and didn't require any other work! So in a square shape around each drain, they cut out and removed all the layers of material down to the metal decking. They installed a new drain and re sealed around it with fabric and coating. From the inside of the building they were able to get at the underside of the flat roof via false office ceilings (with ladders and crawling around) to re connect the underside of the drains with PVC. They had added an extra drain where there wasn't one originally also. No one else was exactly waiting in line to do this. Finally the water on the flat roof drains like it should, like your shower.